The term “comprising” in the present application is intended to convey the idea of a collection of items that are relevant for the present invention, but it does not exclude that further items may be present and/or relevant. The term “comprising” is not intended to convey the idea of a completeness to the exclusion of other items, which would be better described by the expression “consisting of”.
In a backhoe and similar equipment an essential component is a shovel or bucket that is fitted to a stick, the length of which may be changed, and the other end of which is linked to a boom. All movement is usually created by means of hydraulic cylinders. The bucket and its front edge is the interface to the ground that is being modified by the action of the earth moving equipment or contains the earth that is to be moved from one place to the other. The movement of the bucket has to be controlled precisely, frequently with a precision of 0.01 m. The frame of reference is frequently a global grid (GPS) or a local grid, for instance involving rotating laser planes. The depth of the bucket is frequently the most needed measurement. However, it has to be obtained indirectly, from a knowledge of the spatial relationships between the components and from a knowledge of the absolute position of at least one point of one other of these components.
It is impossible to monitor the movement of the front edge of the bucket directly, and instead indirect methods are relied upon. The complexity of the relationships between the various elements in excavating and earth moving equipment is very well illustrated by U.S. Pat. No. 2,538,000, FIGS. 2 and 9. In order to demonstrate the simultaneous movement of several of the moving parts, three trajectories of points on these parts are traced, subdivided in a such a way that equal-numbered subdivisions correspond to simultaneous positions of the various points. The movements are highly non-linear It is, however, possible to calculate geometrically and trigonometrically the position of the edge of the bucket when the angular relationship between all the elements are known, and the length of the stick extension has been taken account of. The angular relationships may be determined directly, by angle encoders on the axes of rotation, and the length of the stick may also be determined directly by linear encoders. Such elements have to be built into the equipment at the time of manufacture, because they are not suitable for retrofitting.
In case of monitoring equipment for retrofitting, angular relationships are frequently obtained indirectly by means of inclination sensors fitted to the various components. The mathematical relationships are used in the processing of the inclination signals obtained and the knowledge about the various distances. Some distances may be measured by means of continuous ultrasound fields, in which an ultrasound transmitter emits a continuous wave, and phase measurements of the signal received by an ultrasound receiver provide the required data. This solution is described for a general application in WO/03088136 A2. In order to provide safety in the movement of the various components of such earth moving equipment, proximity sensors in the form of ultrasound transmitters and receivers ensure that they do not collide, as described in JP2001064992A. A measurement of the extension of a linear ladder is described in WO2010/003636A1, in which an established reference distance is measured by the same ultrasonic elements that measure the extension of the ladder, thereby performing compensation of the influence of temperature and pressure.
Other methods have relied on microwaves or lasers for distance measurements. In particular laser systems are prone to malfunction due to dirt settling on optical surfaces and windows, and due to interruption of line-of-sight between transmitter and receiver.